PROPERTY, PLANT AND EQUIPMENT - Electric Plant (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Mar. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Electric Plant
Accumulated Depreciation		$ (2,805,718)	$ (2,739,924)
Construction work in progress		273,339	163,954
Net electric plant		3,166,515	3,146,709
Electric Plant
Electric Plant
Plant In Service		5,698,894	5,722,679
Accumulated Depreciation		(2,805,718)	(2,739,924)
Net Book Value		2,893,176	2,982,755
Construction work in progress		273,339	163,954
Net electric plant		3,166,515	3,146,709
Generation plant
Electric Plant
Plant In Service		3,098,909	3,082,133
Accumulated Depreciation		(1,721,383)	(1,669,941)
Net Book Value		$ 1,377,526	1,412,192
Generation plant | Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	0.89%	1.14%
Generation plant | Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	6.27%	4.14%
Transmission plant
Electric Plant
Plant In Service		$ 1,929,078	1,983,629
Accumulated Depreciation		(706,983)	(708,412)
Net Book Value		$ 1,222,095	1,275,217
Transmission plant | Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	1.11%	1.17%
Transmission plant | Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	2.09%	1.84%
General plant
Electric Plant
Plant In Service		$ 430,202	410,856
Accumulated Depreciation		(279,264)	(266,013)
Net Book Value		$ 150,938	144,843
General plant | Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	1.46%	1.20%
General plant | Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	9.53%	11.60%
Other
Electric Plant
Plant In Service		$ 240,705	246,061
Accumulated Depreciation		(98,088)	(95,558)
Net Book Value		$ 142,617	$ 150,503
Other | Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	2.75%	2.75%
Other | Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	10.00%	10.00%